The present invention relates to an electromagnetic clutch used as a power transmission device for a lawn mower, snow blower, and the like and, more particularly, to an electromagnetic clutch including a brake mechanism for preventing inertial rotation and idling on the output side after cutting off power transmission.
A conventional electromagnetic clutch of this type has, e.g., a structure as shown in FIG. 5. As shown in FIG. 5, an electromagnetic clutch 1 is mounted on an input shaft 2 extending in the horizontal direction (right-and-left direction in FIG. 5). For descriptive convenience, the distal end side (right side in FIG. 5) of the input shaft 2 will be defined as the front side of the electromagnetic clutch 1, and the opposite side will be defined as the back side of the electromagnetic clutch 1.
The input shaft 2 is driven to rotate by an engine (not shown), a motor (not shown), or the like. A pulley 3 and a rotor 4 of the electromagnetic clutch 1 are mounted on the input shaft 2 to be aligned in the axial direction so that they rotate integrally by so-called key fitting. More specifically, the pulley 3 and rotor 4 have projections 3a and 4a at their inner circumferential portions, and are attached to the input shaft 2 so that the projections 3a and 4a are fitted in a key groove 2a of the input shaft 2.
The pulley 3 transmits power to a power transmission path for driving the wheel (not shown) of the lawn mower. The pulley 3 is positioned on the back side with respect to the rotor 4. The rotor 4 is formed from a boss 5 attached to the input shaft 2, and an annular flange 6 extending outward from the boss 5 in the radial direction. The flange 6 is fitted in the front end of the boss 5. The inner circumferential portion of the flange 6 is partially caulked to clamp the boss 5, and is integrally coupled to the boss 5.
An annular groove 7 which has a U-shaped section and opens toward the back side of the electromagnetic clutch 1 is formed in the flange 6. Also, a plurality of arc slits 8 are formed in the flange 6. The slits 8 are formed to extend through a disc portion 9 forming the bottom of the annular groove 7 so that they extend in the circumferential direction of the flange 6. Two slits 8 are formed side by side in the radial direction at each of a plurality of portions of the disc portion 9 in the circumferential direction.
A field core 12 is supported at the back end of the boss 5 via a bearing 10 and rotation stop plate 11. The annular field core 12 is inserted in the annular groove 7 of the flange 6. An annular groove 12a which has a U-shaped section and opens toward the front side of the electromagnetic clutch 1 is formed in the field core 12. An exciting coil 13 is housed in the annular groove 12a. 
The rotation stop plate 11 regulates rotation of the field core 12. The rotation stop plate 11 includes a cylindrical portion 14 in which the bearing 10 is fitted, and a flat portion 15 welded to a wall forming the bottom of the annular groove 12a in the field core 12. Although not shown, a rotation stop pin extending from a fixed housing is inserted in the rotation stop plate 11.
The cylindrical portion 14 is formed into a cylindrical shape by performing burring work for a plate-like base material. A ring-like abutment plate 16 is welded to the back-side opening portion of the cylindrical portion 14. The abutment plate 16 prevents the bearing 10 from slipping off the cylindrical portion 14. A caulking piece 14a is formed at the front-side opening portion of the cylindrical portion 14. An outer ring 10a of the bearing 10 is clamped by the abutment plate 16 and caulking piece 14a from two sides in the axial direction. That is, movement of the rotation stop plate 11 in the axial direction with respect to the bearing 10 is regulated by the abutment plate 16 and caulking piece 14a. 
An inner ring 10b of the bearing 10 is fitted in a small-diameter portion 5a of the boss 5, and the pulley 3 abuts against the inner ring 10b from the back side. Thus, movement of the inner ring 10b of the bearing 10 in the axial direction is regulated by the pulley 3 and boss 5.
The back end of a cylindrical bearing collar 17 is fitted in and welded to the front end of the boss 5. A hub 19 is rotatably supported by the front end of the bearing collar 17 via a bearing 18. An inner ring 18a of the bearing 18 is pressed backward from the front by a press plate 20. The press plate 20 is pressed against the inner ring 18a by a fixing bolt 21 screwed into the input shaft 2. By screwing the fixing bolt 21 into the input shaft 2, the boss 5 of the rotor 4 and the first pulley 3 are fixed to the input shaft 2.
A second pulley 22 and leaf springs 23 are fixed to the hub 19 by rivets 25a. The second pulley 22 transmits power for driving the blade (blade for cutting a lawn: not shown) of the lawn mower. The free end of the leaf spring 23 is fixed to an armature 24 by a rivet 25. The armature 24 is formed from an annular plate facing the disc portion 9 of the rotor 4, and is supported by the hub 19 via the leaf spring 23. The leaf spring 23 biases the armature 24 in a direction (forward) in which the armature 24 is spaced apart from the rotor 4.
The electromagnetic clutch 1 further includes an annular brake plate 26 which abuts against the front surface of the armature 24 when the exciting coil 13 is in the non-exciting state. The brake plate 26 is supported by the rotation stop plate 11 via an adjusting mechanism 27. The adjusting mechanism 27 changes a position of the brake plate 26 in the front-and-back direction. The adjusting mechanism 27 is made up of a bolt 27a extending through the rotation stop plate 11 and brake plate 26, a helical compression spring 27b which presses forward the brake plate 26, and a nut 27c threadably fixed to the bolt 27a. 
The armature 24 abuts against the inner peripheral portion of the brake plate 26 by the spring force of the leaf spring 23 when the exciting coil 13 is in the non-exciting state. That is, the inner peripheral portion of the brake plate 26 forms a stopper which abuts against the armature 24 that moves apart from the rotor 4 by the elastic return force of the leaf spring 23. Further, the inner peripheral portion of the brake plate 26 forms a brake disk which brakes inertial rotation of the armature 24. As a brake for preventing inertial rotation of the armature 24 after cutting off power transmission, there is a structure which attracts and brakes the armature 24 by using an electromagnet (see Japanese Utility Model Laid-Open No. 59-133845) or a permanent magnet (see Japanese Patent Laid-Open No. 7-190095), in addition to the structure shown in FIG. 5 (structure disclosed in Japanese Patent Laid-Open No. 59-133840).
The position of the brake plate 26 in the front-and-back direction is changed by tightening or loosening the nut 27c of the adjusting mechanism 27. The adjusting mechanism 27 therefore adjusts an air gap formed between the rotor 4 and the armature 24.
When the conventional electromagnetic clutch 1 having this structure is in the non-exciting state in which the exciting coil 13 is not energized, the input shaft 2, rotor 4, and pulley 3 rotate integrally, and rotation of the input shaft 2 is not transmitted to the pulley 22. In the exciting state in which the exciting coil 13 is energized, the armature 24 is magnetically attracted by the rotor 4, and the pulley 22 also rotates together with the input shaft 2.
The above-described conventional electromagnetic clutch 1 has a problem of high manufacturing cost. The manufacturing cost rises because the cost for manufacturing the rotation stop plate 11 which regulates rotation of the field core 12 is high. When manufacturing the rotation stop plate 11, the cylindrical portion 14 is formed by punching a thin plate into a predetermined shape by press work and performing burring work at the center of the punched plate. The inner circumferential surface of the cylindrical portion 14 requires finishing work including cutting work to have a hole for fitting a bearing. Further, the abutment plate 16 formed into a ring shape is welded to the back end of the cylindrical portion 14.
That is, burring work and cutting work for the rotation stop plate 11, welding operation for the rotation stop plate 11 and abutment plate 16, and the like need to be performed. In addition, the abutment plate 16 needs to be manufactured. As a result, the rotation stop plate 11 becomes expensive.